Screen switch: a blubb on every switch, and Del on the kiosk keyboard

ScreenService plays a synthesised two-bubble "blubb" (musicmouse.blubb) after each
switch, through aplay on the music's ALSA device at the player's volume: rising for
on, falling for off, going by the last word the toggle command prints. Nothing plays
with simulated audio or at volume 0. Built from samples rather than shipped as a
file, like the front-end's WebAudio blips, so its loudness follows the volume.

Del maps to a new "screen" action on every page, which App.tsx sends to the new
POST /api/screen/toggle only when the page is served from localhost - the kiosk on
the device - so Del on a tablet does not switch off a screen in another room.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/api/client.ts

@@ -97,6 +97,7 @@ export const api = {
   seek: (position: number) => post("/seek", { position }),
   setVolume: (percent: number) => post("/volume", { percent }),
   nudgeVolume: (deltaPercent: number) => post("/volume", { delta_percent: deltaPercent }),
+  toggleScreen: () => post("/screen/toggle"),
 
   settings: () => request<Settings>("/settings"),
   saveSettings: (settings: Settings) =>